Systems and methods to build  sedimentary attributes

ABSTRACT

A method and system for computing and visualizing sedimentary attributes may include receiving, by a processor, paleo-geographic coordinates representing predicted approximate positions of particles of sediment deposited at a time period when a layer was originally formed. The processor may numerically compute or determine a sedimentation rate that varies laterally along the layer. The processor may determine a sedimentary attribute based on the lateral variation of the sedimentation rate along the layer with respect to the paleo-geographic coordinates. A monitor or display may display the sedimentary attribute of the layer in the present-day geological space.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationNo. 61/790,010 filed on Mar. 15, 2013 and U.S. Provisional PatentApplication No. 61/829,444 filed on May 31, 2013, both of which areincorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The field of the invention relates to characterizing and modelingstratified terrains in the subsurface.

BACKGROUND OF THE INVENTION

Erosion and tectonic activity through geological-time may transform aninitially uniform stratified terrain composed of a continuous stack ofdepositional surfaces, called horizons, into a faulted and foldedterrain fractured by faults forming discontinuities across theoriginally continuous horizons. Accordingly, to model the structures attheir predicted or simulated original time of deposition from datacollected from the current subsurface structures (e.g., to “reversetime”), the model may simulate a reversal of such erosion and tectonicactivity.

A particle of sediment observed today at geographical coordinates (x,y)and altitude or vertical component (z) may have been deposited at anoriginal time of deposition or “geological-time” t(x,y,z) atpaleo-geographical coordinates u(x,y,z) and v(x,y,z). Depositionalcoordinates (u,v,t) so defined may be different than the observed(present day's) geographic coordinates (x,y,z). The “GeoChron” model mayprovide systems and methods to calculate or predict the depositional(past) coordinates u(x,y,z), v(x,y,z) and t(x,y,z) of a particle ofsediment in a subsurface structure from its observed (present day)coordinates (x,y,z) in the geological layers. The depositionalcoordinates (u,v,t) may be displayed to model a simulation of thesubsurface structures as they are predicted to have appeared at theiroriginal time of deposition.

Depending on the paleo-environment prevailing at the geological time ofdeposition, particles of sediment may have been deposited according todifferent depositional styles (such as, for example, on-lap, off-lap, orproportional) which may have impacted the geometry of the layers.

For more than three decades, geologists and geophysicists in the water,mineral (e.g. mining) and energy (e.g. oil and gas) industries havemodelled “attributes” to characterize the properties of sedimentaryterrains in the subsurface. An attribute may be a function f(x,y,z)which, based on observed data, such as seismic data or well data, may beestimated at each location (x,y,z) or coordinate in the present-daysubsurface. The functions may statistically correlate with rock typesand properties of the terrains and the functions may be used to identifythese properties in a current time or predict these properties at a timeof original deposition. These properties may be related to twopreviously known families of attributes, seismic attributes andgeometric attributes:

-   -   Seismic attributes may be defined based on the shape of an        observed seismic signal reflected by geological structures in        the subsurface. The observed seismic signal may be recorded by a        seismograph when measuring ground movement. Frequently used        seismic attributes may include, for example, “instantaneous        amplitude,” “instantaneous velocity,” and “instantaneous        acceleration.” These attributes may be computed using first and        second order derivatives of the observed seismic signal crossing        the geologic layers.    -   Geometrical attributes may be defined based on the shape of        geological horizons or surfaces bounding geological layers and        may provide information about the curvatures and distances to        faults. Frequently used geometrical attributes may include        “Gaussian curvature,” “mean curvature,” and “main curvature.”        These attributes may be computed using first and second order        derivatives of a parametric representation of the horizons        bounding the geologic layers.

SUMMARY OF THE INVENTION

Embodiments of the invention provide a system and method for computingand modeling sedimentary attributes, which describe stratified terrainsor layers of a geological subsurface. Embodiments of the inventionprovide a new family of attributes that may be referred to as“sedimentary attributes.” Sedimentary attributes may define theproperties of the sedimentation process of particles during deposition.Sedimentary attributes may be determined or computed based on lateralvariations of a “sedimentation rate.” The sedimentation rate defines therate at which particles of sediment at their location were deposited.The sedimentation rate is proportional to the thickness of sedimentarylayers. For example, the higher the sedimentation rate, the fasterparticles of sediment were deposited, and the thicker the layer is atthat location. Sedimentary attributes may include, for example,sedimentary expansion (e.g. FIG. 4), sedimentary potential (e.g. FIG.5), sedimentary acceleration (e.g. FIG. 6), sedimentary expansion ratio,sedimentary horizontal divergence, sedimentary horizontal norm, IPG-linecurvature, N-line curvature, and normal divergence. Sedimentaryattributes may be deduced or computed from lateral variations of a“sedimentation rate,” which is itself proportional to the thickness ofgeological layers. For example, the faster particles of sediment weredeposited in geological-time, the thicker its region of the layer.

In practice, each of the sedimentary attributes may be computed based ona lateral variation in sedimentation rate. For example, sedimentaryattributes, such as, sedimentary expansion and sedimentary acceleration,may be computed based on a lateral variation (e.g. first and secondderivatives) of the sedimentation rate through a geological layer.However, in the current geological time, layers become folded and/orfaulted (e.g. see folded layer 302 in the left image of FIG. 3A) and the“lateral” direction (e.g. 318 a, tangential to the curvature of thelayer or horizons bounding the layer) likewise becomes faulted and/orfolded. Therefore, the “horizontal” (x,y) direction in a rectilinear(non-curvilinear) coordinate system (e.g. Cartesian coordinate system)do not reliably conform to the curvature of the layer, but may cross inand out of the layer as it curves along folds or breaks along faults. Toreliably follow the lateral direction within a faulted or folded layer,a second curvilinear coordinate system (e.g. u,v,t) may be used, havingpaleo-geographical axes or “iso-t” surfaces that themselves curve (e.g.see FIG. 7). Iso-t surfaces have a constant (“iso”) geological-time (t)and vary in paleo-geographic coordinates (u,v). Iso-t surfaces representhorizons and other layers, which curve tangentially to the folded and/orfaulted layers when viewed in the original (non-curvilinear) coordinatesystem (e.g. curved iso-surface 318 a in the left image of FIG. 3A).Accordingly, a “lateral” direction following a folded and/or faultedhorizon may be modeled by iso-t surfaces (e.g. 318 b) of the curvilinearcoordinate system (e.g. 310). Therefore, the lateral change insedimentation rate (V(x,y,z)) may be measured along the iso-surfaceswith respect to infinitesimal changes in paleo-geographical coordinate(du and/or dv) of the curvilinear coordinate system (e.g. as dV/duand/or dV/dv).

Sedimentary attributes may also be defined based on a lateral variationin layer thickness. Embodiments of the invention may generate a measureof a thickness Δh of a layer, which due to lateral variations in thesedimentation rate, may vary laterally along the layer in a present daymodel. Variations or changes in the thickness of the layer may be bestmeasured with respect to paleo-geographic coordinates that varylaterally along a depositional surface (also called by geologists an“horizon”) inside the layer, e.g., with respect to (u,v)paleo-geographic coordinates. As shown for example in FIG. 7,paleo-geographic coordinates vary tangentially along the horizon orlayer surfaces regardless of deformation or curvature in the layer. Incontrast, when layers are curved or faulted, modeling the variation oflayer thickness according to a (x,y) geographic coordinates may be lesseffective since the “lateral” or “horizontal” direction in the x-y planemay vary in depth e.g. within a horizon or even cross through severallayers.

As shown, for example in FIG. 2, using the uvt-transform, sedimentaryattributes may be displayed in either a present-day geological timemodel (e.g. in xyz-space) or a depositional model (e.g. uvt-space)visualizing the subsurface layer(s) at a geological time when thestructure was originally deposited.

A three dimensional (3D) depositional model may be represented by forexample functions t(x,y,z), u(x,y,z) and v(x,y,z) such that, for anylocation (x,y,z) in the subsurface:

-   -   t(x,y,z) represents the geological-time of original deposition        of a point representing a particle of sediment observed today at        location (x,y,z);    -   u(x,y,z) and v(x,y,z) represent the paleo-geographic coordinates        at the geological-time of deposition t(x,y,z) of the point        representing the particle of sediment observed today or in the        present-day time at location (x,y,z).

Embodiment of the invention may compute the functions u(x,y,z), v(x,y,z)and t(x,y,z) that model a subsurface structure, such as one or morelayers, or model attributes that are related to the structure. Note thatthese functions may be non-unique (e.g. there are multipletransformations from (x,y,z) coordinates to (u,v,t) coordinates), butmay be considered equivalent when they represent the same equivalentgeometry and deformation of the geological layers transformed to theirpositions as observed today.

In an embodiment of the invention, a computing system or processor maybe used to generate and model sedimentary attributes. The present daycoordinates (x,y,z) may be an “input” of the computing system. Thepresent day coordinates (x,y,z) may be transformed intopaleo-depositional coordinates u(x,y,z), v(x,y,z) and/or t(x,y,z), andmay be used to generate a sedimentary rate V(x,y,z). The sedimentaryattributes SA(x,y,z) may be computed based on the lateral variation ofthe sedimentary rate V(x,y,z) with respect to paleo-depositionalcoordinates u(x,y,z), and v(x,y,z). The “output” of the computing systemmay include the sedimentary attributes SA(x,y,z) modelled in thepresent-day coordinate system (x,y,z) (e.g. as shown in FIGS. 4-6)and/or mapped or transformed to the depositional coordinate system, forexample, by functions SA(u,v,t)=SA(u(x,y,z), v(x,y,z), t(x,y,z)).

BRIEF DESCRIPTION OF THE DRAWINGS

The principles and operation of the system, apparatus, and methodaccording to embodiments of the present invention may be betterunderstood with reference to the drawings, and the followingdescription, it being understood that these drawings are given forillustrative purposes only and are not meant to be limiting.

FIG. 1 is a diagram of a computer system, according to some embodimentsof the invention;

FIG. 2 is a schematic illustration of a transformation, map,parameterization or conversion between a current model in a present-daygeological space and a depositional model in a predicted past geologicalspace, according to some embodiments of the invention;

FIG. 3A is a schematic illustration of a vertical cross section oflayers used to determine a sedimentation rate in the present-daygeological space (left image) and the original depositional geologicalspace (right image), according to embodiments of the invention;

FIG. 3B is a schematic illustration of a vertical cross section of alayer in the present-day geological space used to determine asedimentation rate based on N-lines or IPG-lines, according toembodiments of the invention;

FIG. 4 is a schematic illustration of a model of sedimentary expansionattributes of a horizon, according to embodiments of the invention;

FIG. 5 is a schematic illustration of a model of sedimentary potentialattributes of a horizon, according to embodiments of the invention;

FIG. 6 is a schematic illustration of a model of sedimentaryacceleration attributes of a horizon, according to embodiments of theinvention;

FIG. 7 is a schematic illustration of iso-surfaces of a curvilinearcoordinate system, according to embodiments of the invention; and

FIG. 8 is a flowchart of a method, according to embodiments of theinvention.

For simplicity and clarity of illustration, elements shown in thedrawings have not necessarily been drawn to scale. For example, thedimensions of some of the elements may be exaggerated relative to otherelements for clarity. Further, where considered appropriate, referencenumerals may be repeated among the drawings to indicate corresponding oranalogous elements throughout the serial views.

DETAILED DESCRIPTION OF THE INVENTION

The present description presents a preferred embodiment. The features ofthe invention are subject to alteration and modification and thereforethe present description should not be considered limiting.

Geological layers may include particles of sediment which have beendeposited throughout geological time at paleo-geographic locations.Sedimentary attributes may define or be used to determine parametersrepresenting the sedimentation of particles over time. Sedimentaryattributes may include, for example, sedimentary expansion (FIG. 4),sedimentary potential (FIG. 5), sedimentary acceleration (FIG. 6),sedimentary expansion ratio sedimentary horizontal divergence,sedimentary horizontal norm, curvature of an N-line (FIG. 3B), curvatureof an IPG-line (FIG. 3B), and normal divergence. Numerical method andcomputer systems may be used to derive these attributes from a GeoChronmodel describing the paleo-geographic (Geo) coordinates (u,v) andgeological-time (Chron) coordinate (t) of deposition of the particles ofsediment observed today in the subsurface. The sedimentary attributesmay be determined from seismic data and well data and may be correlatedwith different characteristics or physical properties (e.g.,permeability, porosity, rock type) of the subsurface on a local level,accounting for changes in the properties across the subsurface. From apractical perspective, correlations between sedimentary attributes (e.g.Ricci curvature, sedimentary acceleration) and depositional style (e.g.,sedimentation mode or style) may be used to better estimate thevariations of physical properties in the subsurface (e.g., variationsbetween horizons, faults, and layers). Geological facies of rockformations may strongly depend on sedimentary attributes including theenergy of sedimentary fluxes and, in turn, physical properties of layersmay strongly depend on these facies. As a consequence, sedimentaryattributes may be linked to the physical properties of the terrains andmay be used to guide the modeling of physical properties in thesubsurface.

For the sake of clarity and for the purpose of simplifying thepresentation of embodiments of the invention, the following preliminarydefinitions are given, although other definitions may be used:

Geological-Time

A particle of sediment in a geological terrain may be observed at alocation in the subsurface. The location of the particle may bemathematically represented or modeled, e.g., by a vector, (x,y,z), in athree-dimensional (3D) space, such as the Cartesian coordinate system(of course, when modeling such particles, the position of many particlesmay be modeled together for example using a cell). When modeled, a datastructure such as a node or cell may represent a subset of particles.The time when the particle of sediment was originally deposited may bereferred to as the “geological-time” and may be represented or modeled,e.g., as a geological-time function of the current location of theparticle, t(x,y,z). When used herein, a “current,” “observed” or modernday location for a particle (or data structure representing one or moreparticles) or subsurface feature may mean the location of the item inthe present day, relative to geological time. The actual geological-timeof the deposition of particles may be difficult to determine and may bereplaced, e.g., by any arbitrary monotonic increasing function of theactual geological-time. The monotonic function may be referred to as the“pseudo-geological-time”. Geological-time and pseudo-geological-time arereferred to interchangeably herein.

The geological-time function t(x,y,z) may be monotonic, e.g., thegradient of the geological-time never reduces to the null vector and thegeological-time function has no local maximum or minimum values.

Level Set Surface

Consider a function f(x,y,z) defined in a 3D space in such a way thatits gradient never reduces to the null vector. A level set surface,S(f₀), may be the set of points where the function f(x,y,z) is equal toa given numerical value, f₀.

As an example, if the geological-time t(x,y,z) represents apseudo-geological-time of deposition, then the level set surface H(t₀)of t(x,y,z) may be a geological horizon.

Various mechanisms are currently used for modeling subsurface geologicalterrains:

GeoChron Model, G-space and G-space

When a layer of particles was deposited during a geological-time periodin the past, the layer typically had continuous geometry during thattime period. However, after the passage of time, the layers may becomeeroded and disrupted by faults, for example, resulting from tectonicmotion or other sub-surface movements, which result in uneven anddiscontinuous layers in the present day subsurface. As compared to thecontinuous layers geometry of the past, the discontinuous layers of thepresent are difficult to model. Accordingly, the “GeoChron” model hasrecently been developed to operate between two 3D spaces (e.g. G space104 and G space 106 in FIG. 2). These two 3D spaces or models may be,for example:

-   -   A 3D space G, called the “Geological Space” or G-space,        representing a model of the current subsurface features observed        in modern times or today (e.g., current modeled locations of        particles of sediment in the terrain). The modeled location of        each particle (or of a subset of particles, or of cells        estimating the location of numerous particles) may be        represented by the coordinates (x,y,z), where (x,y) may describe        the today geographical coordinates of the particle (e.g.,        latitude and longitude) and (z) may describe the today altitude        or distance below or above a given reference surface level        (e.g., the sea level); and    -   A 3D space G, called the “depositional-space” or        “parametric-space” or more simply G-space, representing modeled        or predicted locations of particles of sediment at the time when        the particles were originally deposited in the layer. The        modeled location of each particle may be represented by the        coordinates (u,v,t) where (t) may be the geological-time of        deposition of the particle and (u,v) may be the        paleo-geographical coordinates of the particle at        geological-time (t).

The GeoChron model defines a transformation between the two 3D spaces Gand G. The transformation may be referred to as a “uvt-transformation,”for example, transforming the (x,y,z) coordinated of the G space to the(u,v,t) coordinated of the G space. The GeoChron model applies a forwarduvt-transformation to transform the current model (a model of thesubsurface features current in time) in G-space to the originaldeposition model in G-space and applies an inverse or reverseuvt-transformation to transform the original deposition model in G-spaceto the current model in G-space. Accordingly, the GeoChron model mayexecute complex computations on the original deposition model in G-spacewhere geological properties are typically uniform and simple tomanipulate relative to the discontinuous current model in G-space. Oncethe original deposition model in G-space is sufficiently accurate, theGeoChron model may use the “reverse uvt-transformation”to transform themodel, and its geological properties, back to the current time domain inG-space to generate the present day non-uniform geological properties ofthe faulted and eroded present day model.

Embodiments of the invention may compute the functions u(x,y,z),v(x,y,z) and t(x,y,z) at each location (x,y,z) observed (e.g. measuredor interpolated) today in the present-day subsurface model. From apractical perspective, the functions u(x,y,z), v(x,y,z) and t(x,y,z) maybe deduced from seismic data (e.g., a seismic cube) and/or well data(e.g., well markers). The functions u(x,y,z) and v(x,y,z) arecurvilinear (e.g. as shown in FIG. 7), and may be used, for example, tocharacterize curvilinear “lateral” surfaces (iso-t surface shown in inFIG. 7) to determine variations in a layer's thickness or sedimentationrate along the curvilinear iso-t surface.

Reference is made to FIG. 1, which schematically illustrates a systemincluding receiver and computing system in accordance with an embodimentof the present invention. Methods disclosed herein may be performedusing a system 105 of FIG. 6. In other embodiments, methods used hereinmay be performed by different systems, having different components.

System 105 may include a receiver 120, a computing system 130, and amonitor or display 180. The data mentioned herein, e.g., seismic dataand well markers used to form intermediate data and finally to modelsubsurface regions and sedimentary attributes, may be ascertained byprocessing data received by receiver 120 (e.g., a hydrophone ormicrophone). Intermediate data and other data such as results, models,and visualizations, as well as code, instructions or software, may bestored in memory 150 or other storage units. The aforementionedprocesses described herein may be performed by software 160 (e.g.,stored in memory 150 or other units) being executed by processor 140manipulating the data.

Receiver 120 may accept data representing a geo-chronological model. Themodel may include a set of points that depict or represent an estimatedor predicted state of the subsurface structure at a time period when thesubsurface structure was originally formed. The data may be sent to aprocessor 140 to numerically compute sedimentary attributes of asubsurface. The sedimentary attributes may be, for example, a function,a vector field or a tensor field.

Computing system 130 may include, for example, any suitable processingsystem, computing system, computing device, processing device, computer,processor, or the like, and may be implemented using any suitablecombination of hardware and/or software. Computing system 130 mayinclude for example one or more computer controllers or processor(s)140, memory 150 and software 160. Data 155 generated by reflectedsignals, received by receiver 120, may be transferred, for example, tocomputing system 130. The data may be stored in the receiver 120 as forexample digital information and transferred to computing system 130 byuploading, copying or transmitting the digital information. Processor140 may communicate with computing system 130 via wired or wirelesscommand and execution signals.

Memory 150 may include cache memory, long term memory such as a harddrive, and/or external memory, for example, including random accessmemory (RAM), read only memory (ROM), dynamic RAM (DRAM), synchronousDRAM (SD-RAM), flash memory, volatile memory, non-volatile memory, cachememory, buffer, short term memory unit, long term memory unit, or othersuitable memory units or storage units. Memory 150 may storeinstructions (e.g., software 160) and data 155 to execute embodiments ofthe aforementioned methods, steps and functionality (e.g., in long termmemory, such as a hard drive). Data 155 may include, for example, avector field describing a set of paleo-geographic points, instructionsfor processing the collected data to generate a model, or otherinstructions or data. Memory 150 may store a geological-time function, amodel including a set of paleo-geographic coordinates representing astructure when it was originally deposited in the layer (e.g., inuvt-space), a model representing the corresponding structure in acurrent or present-day time period (e.g., in xyz-space), a modelrepresenting a function of one or more sedimentary attributes in thepresent-day or depositional space and/or a model of the physicalproperties (e.g., permeability, porosity, rock type) of the structuresderived from the sedimentary attributes. Memory 150 may store cells,nodes, voxels, etc., associated with the model and the model mesh.Memory 150 may also store forward and/or reverse uvt-transformations totransform current models in xyz-space to models in uvt-space, and viceversa. Data 155 may also include intermediate data generated by theseprocesses and data to be visualized, such as data representing graphicalmodels to be displayed to a user. Memory 150 may store intermediatedata. System 130 may include cache memory which may include dataduplicating original values stored elsewhere or computed earlier, wherethe original data may be relatively more expensive to fetch (e.g., dueto longer access time) or to compute, compared to the cost of readingthe cache memory. Cache memory may include pages, memory lines, or othersuitable structures. Additional or other suitable memory may be used.

Computing system 130 may include a computing module havingmachine-executable instructions. The instructions may include, forexample, a data processing mechanism (including, for example,embodiments of methods described herein) and a modeling mechanism. Theseinstructions may be used to cause processor 140 using associatedsoftware 160 modules programmed with the instructions to perform theoperations described. Alternatively, the operations may be performed byspecific hardware that may contain hardwired logic for performing theoperations, or by any combination of programmed computer components andcustom hardware components. Processor 140 may be configured to carry outembodiments of the present invention by, for example, executinginstructions or code. One or more processors 140 may be configured tocarry out methods of the invention in different manners, for example byincluding specialized circuitry.

Embodiments of the invention may include an article such as a computeror processor readable medium, or a computer or processor non-transitorystorage medium, such as for example a memory, a disk drive, or a USBflash memory, encoding, including or storing instructions, e.g.,computer-executable instructions, which when executed by a processor orcontroller, carry out methods disclosed herein.

Processor 130 may perform various methods described herein. For example,processor 140 may generate a geological time function t(x,y,z) accordingto techniques known in the art. The geological time function t(x,y,z)may be an arbitrary monotonic increasing function of the actualgeological-time.

Processor 140 may generate paleo-geographic coordinates u(x,y,z) andv(x,y,z) and geological-time t(x,y,z) defining a horizon H*(t₀) inG-space, which models a “past” or “paleo” state of the subsurfacestructures at a time when they were originally deposited in the layer.The paleo-geographic coordinates of the past depositional model aregenerated based on the geometry of the current modeled horizons.

Processor 140 may generate functions {u(x,y,z), v(x,y,z), t(x,y,z)}between G-space and G-space to transform a current model in G-space of adiscontinuous faulted horizon to a single substantially continuoushorizon in G-space. The paleo-geographic coordinates {u(x,y,z),v(x,y,z)} may be defined by a system of equations in G-space. Processor140 may determine sedimentary attributes of the functions {u(x,y,z),v(x,y,z), t(x,y,z)} or {x(u,v,t), y(u,v,t), z(u,v,t)} e.g. according toa deposition style. These attributes may include functions ofsedimentation rate, for example, or other attributes based onsedimentation, such as, sedimentary expansion (FIG. 4), sedimentarypotential (FIG. 5), sedimentary acceleration (FIG. 6).

Display 180 may display data from receiver 120, or computing system 130or any other suitable systems, devices, or programs, for example, animaging program or receiver tracking device. Display 180 may include oneor more inputs or outputs for displaying data from multiple data sourcesor to multiple displays. For example display 180 may displayvisualizations of models including sedimentary attributes of subsurfacestructures and/or their subsurface features, such as faults, horizonsand unconformities. Display 180 may display sedimentary attributes ofhorizon H(t₀) in G-space and/or sedimentary attributes of horizon H*(t₀)in G-space. Display 180 may display the models in separate pages orwindows and a user may select one of the models (e.g., by clicking a‘G-space’ or ‘ G-space’ button with a pointing device such as a mouse orby scrolling between the models). A user may select a display option toshow, hide or switch between different sedimentary attributes.

Reference is made to FIG. 2, which schematically illustrates atransformation, map or conversion between a current model 104 and anoriginal depositional model 106 (separated by dashed line 101),according to some embodiments of the invention. Current model 104 mayrepresent the current modeled locations of subsurface structuresincluding particles of sediment in the terrain (typically at agranularity larger than that representing each particle). Current model104 may be a 3D model in a Cartesian (x,y,z)-space, where the locationof each particle is represented by the coordinates (x,y,z), where (x,y)may describe the geographical coordinates of the particle (e.g.,latitude and longitude) and (z) may describe the altitude or distancebelow or above a surface level.

Depositional model 106 may represent estimated or predicted (past)locations of particles of sediment at the time when the particles wereoriginally deposited, for example, in the layer. Depositional model 106may be a 3D model in an (u,v,t)-space, where each particle may berepresented by the coordinates (u,v,t) where (t) may be thegeological-time of deposition of the particle and (u,v) may be thepaleo-geographical coordinates of the particle at geological-time (t).

The “forward” or “direct” transformation 100 may be defined by functions102 {u(x,y,z),v(x,y,z),t(x,y,z)}, which convert or transform each point(x,y,z) of current model 104 to a point {u(x,y,z),v(x,y,z),t(x,y,z)} ofdepositional model 106. The forward transformation 100 may berepresented, for example, as follows:

$\begin{matrix}{\left( {x,y,z} \right)\overset{UVT}{}\left\{ {{u\left( {x,y,z} \right)},{v\left( {x,y,z} \right)},{t\left( {x,y,z} \right)}} \right\}} & \lbrack 1\rbrack\end{matrix}$

The forward transformation or conversion 100 transforms each horizonH(t) (e.g., 108 a or 108 b) of current subsurface structure 108, incurrent model 104, into a level horizontal plane H*(t) (e.g., 110 a and110 b, respectively) of depositional structure 110 in depositional model106. In depositional model 106, horizons 110 a and 110 b of structure110 are simply the images of the level surfaces of the function t(x,y,z)representing the geological-time at location (x,y,z). That is, since ahorizon models a set of particles of sediment that was uniformlydeposited in time, the geological-time of each horizon is constant atthe time when the particles modeled thereby were originally deposited(e.g., in depositional model 106). Therefore, each horizon 110 a and 110b in depositional model 106 may be planar and uniquely defined by asingle geological-time, (t).

Conversely, the “inverse” or “reverse” conversion or transform 112 maybe defined by functions 114 {x(u,v,t), y(u,v,t), z(u,v,t)}, whichtransform each point (u,v,t) of the depositional model 106 to a point{x(u,v,t), y(u,v,t), z(u,v,t)} in current model 104. The inversetransformation 112 may be represented, for example, as follows:

$\begin{matrix}{\left( {u,v,t} \right)\overset{{UVT}^{- 1}}{}\left\{ {{x\left( {u,v,t} \right)},{y\left( {u,v,t} \right)},{z\left( {u,v,t} \right)}} \right\}} & \lbrack 2\rbrack\end{matrix}$

Using the forward transform or conversion 100, e.g., defined in equation(1), and the inverse transform 112, e.g., defined in equation (2), anygeological property may be modeled in one of the two models (currentmodel 104 or depositional model 106) and the result of the propertymodeled in the one space may be transferred to the other space(depositional model 106 or current model 104, respectively). Inpractice, a geological property may be typically modeled in the spacewhere modeling the property is the simplest. For example, horizons maybe modeled in depositional model 106 where they have a simple flatplanar form. Faults may be modeled first in current model 104 since theydid not exist in the original depositional time, and may then betransformed to depositional space, to model their interaction with theplanar horizons in depositional model 106.

Embodiments of the invention may manipulate data representations ofreal-world objects and entities such as underground geologicalstructures, including faults, horizons and other features. Data receivedby for example a receiver receiving waves generated by an air gun orexplosives may be manipulated and stored, e.g., in memory 150, and datasuch as images representing underground structures may be presented to auser, e.g., as a visualization on display 180 in FIG. 1.

Horizons, Faults and Unconformities

In stratified layers, horizons, faults and unconformities may becurvilinear (e.g., non-planar) surfaces which may be for examplecharacterized as follows:

-   -   A horizon, H(t₀), may be a level surface of the geological time        function t(x,y,z) corresponding to a plurality of particles of        sediment which were deposited approximately at substantially the        same geological-time (t₀).    -   A fault may be a surface of discontinuity of the horizons that        may have been induced by a relative displacement of terrains on        both sides of such surfaces. In other words, the geological-time        t(x,y,z) of deposition of the sediments is discontinuous across        each fault. Faults may cut horizons and may also cut other        faults.    -   An unconformity may be a surface of discontinuity of the        horizons that may have been induced by for example an erosion of        old terrains replaced by new ones. In other words, similarly to        faults, the geological-time function t(x,y,z) of deposition of        the sediments is discontinuous across each unconformity. When        discussed herein, unconformities are treated as faults: as a        consequence, in this patent application, faults may include both        real faults and unconformities. Alternately, unconformities may        be surfaces bounding a sequence of sedimentary layers and one        specific geological-time function t(x,y,z) may be assigned to        each such sequence.

Notation

In accordance with the present invention and as used herein, thefollowing notation is defined with the following meanings, unlessexplicitly stated otherwise:

-   -   (grad f) is a vector representing the gradient of a scalar        function f,    -   (a×b) represents the cross product of two vectors (a,b),    -   (a·b) represents the dot product (also called the scalar        product) of two vectors (a,b) and    -   ∥a∥ represents the norm, magnitude, or absolute value of a        vector (a).        (Bold notation generally represents multi-dimensional terms,        such as, vectors.)

Paleo-Geographic Coordinates, IPG-Lines and N-Lines

Each particle of sediment observed today at geographical coordinates(x,y) and altitude (z) may have been deposited at paleo-geographicalcoordinates u(x,y,z), and v(x,y,z) which may differ from the currentgeographic coordinates (x,y). The GeoChron model may provide equationsand algorithms allowing the geological-time function t(x,y,z) to bemodeled at any location (x,y,z) in the subsurface.

For a pair of level surfaces U(u₀) and V(v₀) that are embedded in the 3DG-space:

-   -   U(u₀) may include a plane of points representing particles of        sediment which were deposited at substantially the same        paleo-geographic coordinate (u=u₀). In other words, U(u₀) is a        level surface where the function u(x,y,z) is equal to the        constant value u₀;    -   V(u₀) may include a plane of points representing particles of        sediment which were deposited at substantially the same        paleo-geographic coordinate (v=v₀). In other words, V(v₀) is a        level surface where the function v(x,y,z) is equal to the        constant value v₀;

The intersection of U(u₀) and V(v₀) may be called“Iso-Paleo-Geograhic-Lines” (IPG-lines) which is a line representingparticles of sediments which have been deposited at the samepaleo-geographic coordinates (u₀,v₀). The unit vector T(x,y,z) tangentto the IPG-line passing through location (x,y,z) in the G-space may begenerated, for example, according to the following equation:

T(x,y,z)=(grad u(x,y,z)×grad v(x,y,z))/∥grad u(x,y,z)×gradv(x,y,z)∥  [3]

The geometry of the horizons and the IPG-lines may characterize thegeometry of layers in the subsurface as observed today. Paleo-geographiccoordinates (u,v) may be non-unique and may be rotated and translated togenerate equivalent paleo-depositional coordinate systems in G-space,illustrating that u-lines and v-lines may depend on the choice ofpaleo-geographic coordinates. In contrast, IPG-lines may have anintrinsic geological definition which may not depend on the choice ofpaleo-geographic coordinates (an IPG-line may be a subset of particlesof sediment deposited at the same paleo-location, whatever the system ofpaleo-geographic coordinates). On the one hand, it may be common to usegeometric properties of horizons (e.g., curvatures) as attributes. Bydefinition, a horizon may be a set of particles of sediments which weredeposited at the same geological time regardless of the paleo-geographiccoordinates or system used. In the oil and gas industry, it may becommon practice to compute the curvatures of an horizon and to use themas attributes. On the other hand, an IPG-line is a set of particles ofsediment which were deposited at the same paleo-location whatever thesystem of paleo-geographic coordinates. Thus, it may be possible computethe curvature of such a line and use it as a new attribute.

A curve NL(x₀,y₀,z₀) passing through the point (x₀,y₀, z₀) andconstantly orthogonal to the horizons may be referred to as a“normal-line” also called an “N-line”. The unit vector N(x,y,z) tangentto the N-line passing through location (x,y,z) in the G-space may begenerated, for example, according to the following equation:

N(x,y,z)=grad t(x,y,z)/∥grad t(x,y,z)∥  [4]

Similar to what was described above for IPG-lines, it be possible tocompute the curvature of N-lines and use them as attributes. IPG-linesgenerally differ from N-lines, but both may be invariant under anyequivalent transformation or parameterization.

Computing Derivatives

A 3D space S, where each point of this space is represented by threecoordinates (X,Y,Z), may be defined by the GeoChron model as follows:

-   -   If (X,Y,Z) is identified with present-day coordinates (x,y,z),        then the space S is identical to the G-space (e.g. current model        104 of FIG. 1).    -   If (X,Y,Z) is identified with paleo-coordinates (u,v,t), then        the space S is identical to the G-space (e.g. depositional model        106 of FIG. 1).

Embodiments of the invention may be used to numerically compute thepartial derivatives of any functions F(X,Y,Z) at any location (X,Y,Z) ofa 3D space S. For example, assuming that F(X,Y,Z) has been sampled atthe nodes (X_(i),Y_(j),Z_(k)) of a 3D regular rectilinear grid withhexahedral cells and steps ΔX, ΔY and ΔZ in the (X), (Y) and (Z)directions, a finite difference method or other method may be used tocompute the partial derivatives of F(X,Y,Z) as follows:

dF(X _(i) ,Y _(j) ,Z _(k))/dX={F(X _(i+1) ,Y _(j) ,Z _(k))−F(X _(i−1) ,Y_(j) ,Z _(k))}/{2ΔX}

dF(X _(i) ,Y _(j) ,Z _(k))/dY={F(X _(i) ,Y _(j+1) ,Z _(k))−F(X _(i) ,Y_(j−1) ,Z _(k))}/{2ΔY}

dF(X _(i) ,Y _(j) ,Z _(k))/dZ={F(X _(i) ,Y _(j) ,Z _(k+1))−F(X _(i) ,Y_(i) Z _(k−1))}/{2ΔZ}

A person of ordinary skill in the art may appreciate that these formulasmay straightforwardly be generalized to numerically compute higher orderderivatives with respect to X, Y, and Z.

Embodiments of the invention may also numerically compute derivatives offunctions sampled at the nodes (also called vertices) of irregular gridssuch as, for example but not limited to, grids with tetrahedral cellsand, more generally, polyhedral cells. After estimating derivatives atsampling locations (X_(i),Y_(j),Z_(k)), local interpolations may then beused to numerically estimate these derivatives at any location withineach cells.

These classical numerical methods may be used to numerically computepartial derivatives of any order. In particular, but not limited to, forany function F(X,Y,Z), these methods may be used to compute:

-   -   a gradient denoted “grad F” at each location (X,Y,Z),    -   a Laplacian denoted “Δ F” at each location (X,Y,Z),    -   a directional derivative dF/ds of each function F(X,Y,Z) with        respect to the arc length (s) in a given direction parallel to a        unit vector D. Directional derivative dF/ds may be defined by        the dot product of the gradient of F(X,Y,Z) by D, for example,        as follows:

dF/ds=grad F(X,Y,Z)·D

In the case where W(X,Y,Z) is a vector field, the partial derivatives ofthe components W_(X)(X,Y,Z), W_(Y)(X,Y,Z) and W_(Z)(X,Y,Z) of W(X,Y,Z)may be numerically computed. In such a case, using these numericalderivation techniques, the divergence {div W(X,Y,Z)} of W(X,Y,Z) mayalso be computed, for example, as follows at each location (X,Y,Z):

div W(X,Y,Z)=dW _(X) /dX+dW _(Y) /dY+dW _(Z) /dZ

More generally, any scalar, vectorial or tensorial function involvingderivatives of W(X,Y,Z) or its components may be computed.

Sedimentary Attributes

Based on the systems and methods described above, the functionsu(x,y,z), v(x,y,z) and t(x,y,z) may be computed at any location (x,y,z)observed today in the subsurface. Each of these functions may benon-unique, variations of which may be considered as equivalent when thevariations equivalently represent the same geometry and deformation ofthe geological layers, for example, when transformed to the currentmodel observed today. For example, if u(x,y,z), v(x,y,z) and t(x,y,z)are equivalent to u*(x,y,z), v*(x,y,z) and t*(x,y,z), then:

-   -   the horizons corresponding to the level set surfaces of the        geological-time functions t(x,y,z) and t*(x,y,z) may have the        same geometry, such as, thickness or curvature;    -   the IPG-lines corresponding to the intersection of the level set        surfaces of the paleo-geographic functions u(x,y,z) and        v(x,y,z), respectively, may have the same geometry;    -   the N-lines orthogonal to the horizons may have the same        geometry; and/or    -   the strain tensor or deformation calculated or deduced from the        functions u(x,y,z), v(x,y,z) and t(x,y,z) may be identical to        the strain tensor or deformation calculated or deduced from the        functions u*(x,y,z), v*(x,y,z) and t*(x,y,z).

Sedimentary attributes may be represented by a function SA(x,y,z), whichis determined, calculated or deduced using one or more of the functionsu(x,y,z), v(x,y,z) and t(x,y,z). The function SA(x,y,z) may be furthertransformed or parameterized and described by the functionSA(u,v,t)=SA(x(u,v,t),y(u,v,t),z(u,v,t)). For example, a sedimentaryattribute may be based on sedimentation rate, as further explainedbelow. The sedimentary attributes SA(x,y,z) may be derived orrepresented in G-space (e.g., describing sedimentary attributes of thecurrent subsurface structure) and may be transformed to representsedimentary attributes SA(u,v,t) in G-space (e.g., describingsedimentary attributes of the subsurface structure at a time of originaldeposition).

In FIG. 2, the (u,v,t) space 106, a top horizon 110 a and a bottomhorizon 110 b may bound a layer in the depositional model. Thesehorizons 110 a and 110 b may be parallel horizontal surfaces forming alayer in G-space 106 and may correspond to folded or faulted horizons108 a and 108 b forming a layer in G-space 104. Folded or faultedhorizons 108 a and 108 b may be parallel bounding a layer of laterallyconstant thickness or non-parallel bounding a layer of laterally varyingthickness. One of these layers 302 is shown in G-space in FIG. 3A thathas laterally varying thickness. In some embodiments, sedimentaryattributes, such as, derivatives and other functions of sedimentationrate, may be determined based on the laterally variation in thethickness or the associated sedimentary rate of the layers 302. Thesesedimentary attributes may be visualized in present-day model 108 orpast depositional model 110 or may be used to determine geometricproperties of the models.

Reference is made to FIG. 3A, which schematically illustrates a layer302 in G-space and its image 304 in G-space used to determinesedimentary attributes, according to embodiments of the invention. FIGS.3A and 3B each show a 2D vertical cross-section of, but represent, a 3Dmodel. Depending on the axes in which the cross-sections are taken,G-space may be represented by a (x,z) and/or (y,z) cross-section andG-space may be represented by a (u,t) and/or (v,t) cross-section.

In FIG. 3A, layer 302 in G-space (e.g., representing a model of thecurrent subsurface features observed in modern times or today) may betransformed to a layer 304 in G-space (e.g. representing a depositionalmodel). Layer 302 may have a thickness Δh bounded by a bottom horizonH_(t) 306 and a top horizon H_(t+Δt) 308, deposited at geological-timest and t+Δt, respectively. The thickness Δh of layer 302 may benon-constant, varying laterally along layer 302. As shown, layer 304 inG-space has a constant (Δt) along each horizon, e.g. since the particlesof each horizon are deposited at substantially the same geological time.In G-space, horizontal motion changes only in the (x,y) 314 plane, butnot in the (z) axis 316. However, lateral motion along layer 302 changesnot only in the (x,y) 314 plane but also along the (z) axis 316.Therefore, to determine lateral variations in sedimentation rate (e.g.to generate sedimentary attributes), embodiments of the invention maydetermine lateral variation in sedimentation rate (or thickness Δh)based on change (e.g. derivatives) of the sedimentation rate withrespect to the paleo-geographic coordinates (u,v). The paleo-geographiccoordinates vary along a surface 318 that follows the curvature ofhorizons bounding layer 302. Since layer 302 is folded, the shape ofhorizons 306 and 308 as well as intermediate surface 318 a in G-spacevaries along all dimensions (x,y,z) of the present-day G-space. Sincethe curvilinear coordinates (u,v) in the G-space follow the curvature ofa horizon 318 a in the layer, it may be easier to determine lateralchange or variation in the direction of the curvilinear (u,v)coordinates in G-space, as shown by surface 318 b. As with horizons 306and 308, surface 318 b may be an iso-t surface in which paleo-geographiccoordinates (u,v) vary, but the geological-time of deposition (t)remains constant. Therefore, surface 318 b may follow the curvature ofhorizons 306 and 308. Thus, the lateral change in sedimentation rate maybe measured as a rate of lateral change within a layer, at a constantgeological time, e.g., without crossing through different layersdeposited at different geological times.

In a demonstrative example, a layer with constant thickness may bemodeled to behave similarly to a book with a fixed number of pagesbounded by a first page (e.g. representing horizon H_(t) 306) and a lastpage (e.g. representing horizon H_(t+Δt) 306). Whether the layer isfolder or unfolded, its thickness may be the length of a segmentorthogonal to H_(t) 306 and H_(t+Δt) 308 and may be equal to the sum ofthe thicknesses of the pages or layers. In other words, if there issubstantially no compaction and the layers can slide on each otherwithout generating gaps (e.g., like the pages of a book bending andsliding against each other without creating gaps) according to a“flexural slip” tectonic style of deformation, the thickness of thelayers, Δh, may be invariant throughout geological-time in a curvilinearcoordinate system (u,v,t). However, contrary to the pages of a book, thethickness of real geological layers may vary laterally with respect to(u,v,t). For example, layer 302 in FIG. 3A has a non-constant thickness.

When the thickness of layer 302 Δh becomes infinitely small, thegradient of the geological-time function t(x,y,z) at location (x,y,z)may be a vector of the G-space such that:

grad t(x,y,z)=(Δt(x,y,z)/Δh(x,y,z))·N(x,y,z)  [5]

where N(x,y,z) 312 may be the unit normal vector orthogonal to Ht atlocation (x,y,z) positively oriented in the direction of the youngerterrains, e.g. as defined in equation (4). The sedimentation rate may bea scalar function V(x,y,z) at any location (x,y,z) in the geologicalspace as observed today. The sedimentation rate may be equal orproportional to the speed or rate at which sediment is deposited on theEarth's surface throughout geological time. Accordingly, thesedimentation rate is proportional to the layer's thickness, Δh. Thesedimentation rate may be defined for example as:

V(x,y,z)={1/∥grad t(x,y,z)∥}=limit Δh/Δt when Δt goes to zero  [6]

This function represents an “apparent” or approximate sedimentationrate, and represents a “real” sedimentation rate for example when thereis substantially no compaction and if the times of deposition assignedto the horizons are the actual or observed geological-times (e.g.experimentally verified with carbon dating or other techniques).

Embodiments of the invention may account for compaction of layers, whichmay be a process by which the porosity of a layer of sediment isdecreased, or the compression or density of the sediment is increased,for example, as a result of its particles being squeezed together by theweight of overlying layers of sediment or by mechanical means. Layers ofsediment that overlie or cover another layer may be layers deposited ata time later than the layers below (e.g., having a smaller z-value in(x,y,z)). A column of sediment h(x,y,z) orthogonal to the horizonsobserved today at location (x,y,z) in the G-space between two horizonsH_(t) and H_(t+Δt) may have a compacted vertical thickness Δh_(C)(x,y,z)at geological-time t of deposition such that:

Δh(x,y,z)=(1−C(x,y,z))·Δh _(C)(x,y,z)

with:0≦C(x,y,z)<1.  [7]

where the compaction function C(x,y,z) may be a compaction coefficientand may e.g. depend on the trajectory of the particles of sedimentthroughout geological-time and/or the nature of the sediment observed atlocation (x,y,z). The compaction function C(x,y,z) may vary laterally asdifferent regions of the layer are more or less compacted. Based onequations (6) and (7), a sedimentation rate that takes compaction intoaccount may be defined, for example, as:

V(x,y,z)={1/∥grad t(x,y,z)∥}/{1−C(x,y,z)}  [8]

A relatively high velocity of deposition (e.g., high sedimentation rate)at a point (x,y,z) may indicate a greater thickness of the layersurrounding that point.

Sedimentary Attributes Computed Based on the Lateral Variation inSedimentation Rate

The sedimentation rate V(x,y,z) may be used to determine or calculatesedimentary attributes that characterize the shape of the layers basedon lateral variations of the sedimentation rate (e.g., as shown in FIG.7, the lateral curvilinear directions u and v being tangent to thehorizons). For example, throughout geological time, horizons may beimpacted and shifted by tectonic forces and, as observed today, are nowfolded and faulted. From a geo-mechanical perspective, at any locationor point (x,y,z) in the current geological G-space, these tectonictransformations may be characterized by strain or relative displacementof the particle (x,y,z). The strain at location (x,y,z) may be definede.g. by a strain tensor E(x,y,z). The deformations may be modeled by astrain tensor E(x,y,z) and the directions of the potential fractures atthat location (x,y,z) may be defined by the Eigen vectors of the straintensor. The strain tensor may characterize the total deformations of thestack of horizons subjected to tectonic forces. A potential direction offracture at location (x,y,z), characterized by strain tensor E(x,y,z)may indicate that if a fracture occurs at location (x,y,z), thedirection of this fracture should be a function of the eigen vectors ofE(x,y,z). In terms of the GeoChron model, at any location (x,y,z) in theG-space, the strain tensor E(x,y,z) may be a function of thepaleo-geographic coordinates u(x,y,z) and v(x,y,z) in G-space space.Therefore, the directions of potential fractures E(x,y,z) may beindirectly controlled by the functions u(x,y,z) and v(x,y,z). Straintensors may measure how far the stacked horizons differ from flat planesand how the horizons moved relatively to each other during tectonicevents.

Attributes based on the lateral variation in sedimentation rate mayinclude, for example, sedimentary expansion (FIG. 4), sedimentarypotential (FIG. 5), and sedimentary acceleration (FIG. 6), as describedbelow.

Sedimentary Expansion Attribute

The sedimentary expansion attribute may represent or describe variationsin the flux or flow of geological materials (e.g., sediment) as theywere deposited. Flux may generally describe the flow or movement ofphysical material through space. Deformations of horizons induced bylateral variations of layers thickness may also be viewed as sedimentarydeformations related to variations of the flux of geological materialsat depositional time and characterized by the “Ricci curvature tensor”which is a function of the second order derivatives of the sedimentaryrate. Contrary to the mechanical strain tensor, the Ricci tensor mayquantify how far the parallelism of horizons is impacted by thevariations or changes of sedimentary fluxes and may not depend ontectonic forces. The Ricci scalar curvature KR may be the sum of all theeigen values of the Ricci curvature tensor and may be determined orcalculated, for example, by partial derivatives of the sedimentationrate with respect to (u) and (v), as shown below.

KR(x,y,z)=−2{d ² V(x,y,z)/du ² +d ² V(x,y,z)/dv ² }/V(x,y,z)  [9]

As equation (9) shows, this Ricci scalar curvature KR may be defined byvariations (derivatives such as first or second order derivatives) inthe lateral thickness of the layers in curvilinear directions (u,v)tangent to the horizons (e.g. along iso-t surface 700 in FIG. 7).

Similar to this Ricci scalar curvature KR(x,y,z), a sedimentaryexpansion rate THETA may be determined to locally characterize ordescribe the deformation induced by lateral variations of thesedimentation rate.

THETA(x,y,z)=−KR(x,y,z)/constant  [10]

where, e.g., constant is 30. The sedimentary expansion may measure apart of the dilatation of the terrains induced by the variations of theflux of geological materials along the horizons. This sedimentaryexpansion may be independent of the “mechanical dilatation” induced bytectonic forces associated to the “geo-mechanical” strain tensorE(x,y,z) explained above.

Reference is made to FIG. 4, which schematically illustrates a model ofsedimentary expansion attribute THETA(x,y,z) of a horizon, according toembodiments of the invention. Darker areas 402 in FIG. 4 may correspondto dilatations in the subsurface and lighter areas 404 in FIG. 4 maycorrespond to contractions in the subsurface. In some embodiments, thesedimentary expansion attribute may be used to indicate the expansion ofthe thickness in the lateral direction e.g. tangential to the horizon.For example, in the oil and gas industry, that expansion may becorrelated with the variation of the rock type and geo-physicalproperties.

Sedimentary Potential Attribute

Using the sedimentation rate V(x,y,z) and classical derivation rules,the Ricci scalar curvature described by equation (9) may also beequivalently expanded as follows:

−(½)KR(x,y,z)=d ² Log {V(x,y,z)}/du ² +d ² Log {V(x,y,z)}/dv ²+(d Log{V(x,y,z)}/dv)²+(d Log {V(x,y,z)}/dv)²  [11]

As equation (11) shows, Log {V(x,y,z,)} may be used to determine theRicci scalar curvature KR(x,y,z). Therefore, Log {V(x,y,z,)} may be usedto represent a sedimentary potential S(x,y,z)

S(x,y,z)=Log {V(x,y,z)}  [12]

Substituting Log {V(x,y,z)} with S(x,y,z), the Ricci scalar curvaturedefined by equation [9] or [11] may be rewritten as:

−(½)KR(x,y,z)=d ² S(x,y,z)/du ² +d ² S(x,y,z)/dv ² +{dS(x,y,z)/dv} ²+{dS(x,y,z)/dv} ²

A potential may be a scalar function, the gradient of which is used todefine a vector field. As shown further below, a sedimentaryacceleration A(x,y,z) may be a vector field related to the lateralvariation in the sedimentation rate (and further, related to the lateralvariation in the thickness of the layers) gradient of the sedimentarypotential S(x,y,z). Due to this relationship between S(x,y,z) andA(x,y,z), both S(x,y,z) and A(x,y,a) may charachterize the sedimentationprocess. In some embodiments, for example, the sedimentary potentialattribute may be correlated with rock types at a geological time whenthe particles were originally deposited.

Reference is made to FIG. 5, which schematically illustrates a model ofsedimentary potential attribute S(x,y,z) of a horizon, according toembodiments of the invention. Some areas may be related to highpotential areas 504 and other areas may be related to low potentialareas 502.

Sedimentary Acceleration Attribute

The sedimentary acceleration attribute may be a vector field which istangent to the horizons and which may have the property of canceling outat locations where the sedimentation rate is constant. The vector fieldmay have a magnitude proportional to the rate of variation in thethickness of the layer and a direction oriented toward the direction ofincreasing layer thickness. For example, the larger the sedimentaryacceleration vector, the faster the layers may thicken in the directionof the vector. The sedimentary acceleration vector field A(x,y,z) may bedefined, for example, as the gradient of scalar function S(x,y,z) withrespect to the (u,v) iso-t surface:

A(x,y,z)={dS(x,y,z)/du}·Ru(x,y,z)+{dS(x,y,z)/dv}·Rv(x,y,z)  [13]

In this definition, Ru(x,y,z) and Rv(x,y,z) may be vector fieldsembedded in the G-space tangential to the horizon at location (x,y,z)and defined as follows:

Ru(x,y,z)=J(x,y,z)·{grad v(x,y,z)×grad t(x,y,z)}

Rv(x,y,z)=J(x,y,z)·{grad t(x,y,z)×grad u(x,y,z)}

The Jacobian function J(x,y,z) associated with the uvt-transform may bedefined, for example, as follows:

J(x,y,z)=1/[{grad u(x,y,z)×grad v(x,y,z)}·grad t(x,y,z)]  [14]

Reference is made to FIG. 6, which schematically illustrates a model ofsedimentary acceleration attribute A(x,y,z) of a horizon, according toembodiments of the invention. As shown, attribute A(x,y,z) is a vectorfield 602 tangent to the horizons and oriented in the direction ofincreasing layer thickness. The magnitude of the vectors in 602∥A(x,y,z)∥ is proportional to the variation of the sedimentation rateV(x,y,z) in the direction of A(x,y,z). Thus, the larger ∥A(x,y,z)∥ is,the faster the layers thicken in direction A(x,y,z).

In some embodiments, the sedimentary acceleration attribute may be usedto indicate which direction has the greater increase in thickness andtherefore, the direction from where the sediments flowed at time of whenthe sediments in the layer were originally deposited. For example, inthe oil and gas industry, that original direction may show a pathfollowed by organic sediments, which generated the oil or gas.Therefore, these sedimentary attributes can be used by explorers to findoil or gas.

Additional Attributes

Other sedimentary attributes may be derived or determined based on thesedimentation rate or sedimentary acceleration. For example:

-   -   1. An expansion ratio Ep(x,y,z) may be defined as follows:

Ep(x,y,z)=J(x,y,z)/V(x,y,z)  [15]

The expansion ratio may be based on a cubic dilatation that describesthe relationship between volume of a unit of sediment observed today andvolume of the unit of sediment at a time that the sediment wasoriginally deposited. The expansion ratio Ep(x,y,z) may be defined asthe ratio of an infinitesimal element of volume of sediments dM(x,y,z)observed today and the same infinitesimal element of volumedM_(d)(x,y,z) at depositional time.

Ep(x,y,z)=dM(x,y,z)/dM _(d)(x,y,z)

If the volume of a unit of sediment observed today is equal to thevolume of a unit of sediment at depositional time, then the expansionratio is equal to one. The ratio of dM(x,y,z)/dM_(d)(x,y,z) may be theexpansion ratio and may be approximated, for example, asJ(x,y,z)/V(x,y,z), where J is the Jacobian function and V is thesedimentation rate.

-   -   2. A “sedimentary horizontal divergence” HD(x,y,z) of A(x,y,z)        may be defined by:

$\begin{matrix}\begin{matrix}{{{HD}\left( {x,y,z} \right)} = {{div}\left\lbrack {A\left( {x,y,z} \right)} \right\rbrack}} \\{= {{{^{2}{Log}}\; {{V\left( {x,y,z} \right)}/{u^{2}}}} + {{^{2}{Log}}\; {{V\left( {x,y,z} \right)}/{v^{2}}}}}} \\{= {{{^{2}{S\left( {x,y,z} \right)}}/{u^{2}}} + {{^{2}\left( {x,y,z} \right)}/{v^{2}}}}}\end{matrix} & \lbrack 20\rbrack\end{matrix}$

-   -   3. A square of the “sedimentary horizontal norm” HN(x,y,z) of        A(x,y,z) may be defined by:

HN ²(x,y,z)={dS(x,y,z)/du} ² +{dS(x,y,z)/dv} ²

-   -   which may be expanded to:

HN ²(x,y,z)={d Log V(x,y,z)/du} ² +{d Log V(x,y,z)/dv} ²  [21]

Reference is made to FIG. 3B, which schematically illustrates a verticalcross section of a layer 302 in the present-day geological space used todetermine a sedimentation rate based on the geometry of N-lines 320 (orequivalently the geometry of IPG-lines), according to embodiments of theinvention. In the following description, N-lines are discussed, althoughthe same discussion relates to IPG-lines.

Sedimentary attributes may be determined based on the curvature ofN-lines 320, which may be correspond to the lateral variations ofsedimentation rate V(x,y,z) along a layer. For example, if thesedimentation rate V(x,y,z) is constant laterally along layer 302 in acurrent geological time (G-space), sediment is deposited at the samerate along the layer and the layer will have the same thicknessthroughout. When layer 302 is folded or deformed over time, layer 302becomes curved, but its thickness remains the same. Accordingly, thecurvature of top horizon 306 approximately matches the curvature of thebottom horizon 308, only shifted vertically upwards (i.e. the top andbottom horizons are parallel). Because the top and bottom horizons 306,308 are approximately parallel, e.g. as shown in the right-side segmentof layer 302 in FIG. 3B, normal vectors 312 a and 312 b may be orientedsubstantially or approximately identically in the same direction. Thus,an N-line 320 a (or IPG-line) connecting those normal vectors 312 a and312 b (or IPG-vectors) may be defined by a straight N-line 320 a. In oneexample (not shown), the top and bottom horizons bounding the layer in acurrent geological time may be parallel flat planes (e.g., parallel tothe (x,y) plane). In this example, N-lines may be straight linesparallel to the z-axis 316. In another example (shown in FIG. 3B), thetop and bottom horizons 306, 308 of layer 302 may be parallel, butcurved instead of flat. In this example, N-line 320 a of the top andbottom horizons may be a straight line connecting normal vectors 312 aand 312 b of the top and bottom horizons. Unlike the first instance,N-line 320 a may not be parallel with the z-axis, but may neverthelessbe a straight line. In contrast, when the sedimentary rates variesthroughout layer 302 (e.g. as shown in the left-side by the bulgingsegment of the layer in FIG. 3B), the curvature of top and bottomhorizons 306, 308 are not parallel, and their corresponding normalvectors 312 c and 312 d are oriented in different directions.Accordingly, an N-line 320 b connecting those N-vectors will be curved.Thus, a curved N-line 320 b may indicate that horizons 306, 308 boundinglayer 302 are not parallel and that the thickness (and thussedimentation rate) of layer 302 varies laterally along the layer (e.g.along surface 318 a). The curvature of N-line 320 b may thereforeindicate the presence of lateral variations in the sedimentation ratealong layer 302 (whereas a straight N-line having zero-curvature 320 amay indicate a constant sedimentation rate along layer 302). Since thepresence of lateral variations of sedimentation rate V(x,y,z) may bedescribed by the curvatures of N-lines 320 (or similarly IPG-lines orother parameters computed from the curvature of N-lines and IPG-lines),these curvatures may be sedimentary attributes. Sedimentary attributesdescribed by N-line or IPG-line curvatures may be defined, for example,as follows:

-   -   1. The IPG-line curvature vector field k_(ipg)(x,y,z) may be        defined as follows where (s) is the arc-length abscissa along        the IPG-line passing through the point (x,y,z) and        T_(ipg)(x,y,z) is the unit vector tangent at this IPG-line at        point (x,y,z):

k _(ipg)(x,y,z)=dT _(ipg)(x,y,z)/ds  [16]

-   -   -   In addition, the following functions may also be sedimentary            attributes:            -   a. the module ∥k_(ipg)(x,y,z)∥            -   b. the divergence Dk_(ipg)(x,y,z)=div {k_(ipg)(x,y,z)}

    -   2. The N-line curvature vector field k_(n)(x,y,z) may be defined        as follows where (s) is the arc-length abscissa along the N-line        passing through the point (x,y,z) and N(x,y,z) is the unit        vector tangent at this N-line at point (x,y,z):

k _(n)(x,y,z)=dN(x,y,z)/ds  [17]

-   -   -   In addition, one can also consider the following functions            as Sedimentary attributes:            -   a. the module ∥k_(n)(x,y,z)∥            -   b. the divergence Dk_(n)(x,y,z)=div {k_(n)(x,y,z)}

    -   3. The normal divergence ND(x,y,z) may be defined as the        divergence of the field of unit normal vectors N(x,y,z) in the        G-space.

$\begin{matrix}{\mspace{20mu} {\text{?}{\text{?}\text{indicates text missing or illegible when filed}}}} & \lbrack 18\rbrack\end{matrix}$

Any particle of sediment observed today in the present-day G-space maybe equivalently characterized by the three parameters (x,y,z) or thethree parameters (u,v,t). As a consequence, using the uvt-transformdescribed in FIG. 2, any sedimentary attribute SA(x,y,z) function of(x,y,z) may be also transformed or parameterized as a sedimentaryattribute SA(u,v,t)=SA(u(x,y,z), v(x,y,z), t(x,y,z)) function of(u,v,t); conversely the reverse uvt-transformation or parameterizationmay apply. For example, the sedimentation rate defined by equation (6)or (8) may be uvt-transformed between xyz-space and uvt-space:

V(u,v,t)=V{u(x,y,z),v(x,y,z),t(x,y,z)}=V(x,y,z)  [19]

Whether a scalar, vectorial or tensorial function, a sedimentaryattribute SA(x,y,z) may be visualized and displayed, e.g. as shown inFIGS. 4-6. Sedimentary attribute SA(x,y,z) may be displayed on a horizonH(t) corresponding to the set of particles of sediments which wereapproximately deposited at a common geological time (t). Alternately,sedimentary attribute SA(x,y,z) may be displayed on any cross section(iso-surface) of a model (e.g. iso-x, iso-y, iso-z, iso-u, iso-v,iso-t), or any other surface or volume. For example, the cross sectionmay be a vertical cross-section or a horizon surface.

Reference is made to FIG. 7 illustrating iso-surfaces of a curvilinearcoordinate system, according to embodiments of the invention.Embodiments of the invention may map a model of a layer bound orpositioned between horizons (e.g., 306 and 308 in FIG. 3A or 108 a and108 b in FIG. 2) from a non-curvilinear first (x,y,z) coordinate systemto a curvilinear second (u,v,t) coordinate system 704. Thenon-curvilinear coordinate system may be rectilinear, for example, andmay include axes that are mutually perpendicular, such as in theCartesian coordinate system. In contrast, the axes of the curvilinearcoordinate system are not mutually perpendicular, but curve, as theymonotonically increase (e.g., excluding spherical or other periodicallyrepeating coordinates systems). A sedimentation rate may be determinedthat varies laterally along the layer. One or more sedimentaryattributes based on the lateral variation of the sedimentation ratewithin the layer may be determined by approximating the change in thesedimentation rate along one or more iso-t surfaces 700 of thecurvilinear second coordinate system 704. Iso-u 706, iso-v 702 and iso-t700 surfaces may be surfaces having a single (iso) value (u), (v), and(t), respectively, and the remaining coordinates vary. For example,iso-t surface 700 may have a single (t) value, but a range or all (u)and (v) values. Iso-t surface 700 may define the lateral directionfollowing the curvature of a sedimentary layer along which sedimentaryattributes are computed. The sedimentary attributes may be displayed ina model in the curvilinear coordinate system 704 or non-curvilinearcoordinate system (e.g., 106 in FIG. 2).

FIG. 8 is a flowchart of method, according to embodiments of theinvention.

In operation 802, a receiver or processor may receive paleo-geographiccoordinate functions representing predicted approximate positions ofparticles of sediment deposited at a time period when a layer wasoriginally formed. The paleo-geographic coordinate functions may becharacterized by u(x,y,z), v(x,y,z) and/or t(x,y,z).

In operation 804, a processor (e.g., 120 of FIG. 1) may determine asedimentation rate that varies laterally along the layer. Thesedimentation rate may be a scalar function that is proportional to thespeed or rate at which sediment is deposited on the Earth's surface.

In operation 806, a processor may determine a sedimentary attributebased on the lateral variation of the sedimentation rate along the layerwith respect to paleo-geographic coordinates (e.g. along an iso-tsurface). The lateral variation may depend on derivatives of thesedimentation rate with respect to the paleo-geographic coordinates (u)and (v). The sedimentary attributes may include sedimentary accelerationor sedimentary expansion, for example.

In operation 808, a display unit (e.g., 150 of FIG. 1) may display asedimentary attribute of the layer in the present-day geological spaceor depositional space. Attributes may be trsnformed back and forthbetween the two spaces.

Embodiments of the invention may include an article such as a computeror processor readable non-transitory storage medium, such as for examplea memory, a disk drive, or a USB flash memory device encoding, includingor storing instructions, e.g., computer-executable instructions, whichwhen executed by a processor or controller, cause the processor orcontroller to carry out methods disclosed herein. Different embodimentsare disclosed herein. Features of certain embodiments may be combinedwith features of other embodiments; thus certain embodiments may becombinations of features of multiple embodiments. The foregoingdescription of the embodiments of the invention has been presented forthe purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed. Itshould be appreciated by persons skilled in the art that manymodifications, variations, substitutions, changes, and equivalents arepossible in light of the above teaching. It is, therefore, to beunderstood that the appended claims are intended to cover all suchmodifications and changes as fall within the true spirit of theinvention.

What is claimed is:
 1. A method to visualize sedimentary attributes, themethod comprising: receiving paleo-geographic coordinates representingpredicted approximate positions of particles of sediment deposited at atime period when a layer was originally formed; determining asedimentation rate that varies laterally along the layer; determining asedimentary attribute based on the lateral variation of thesedimentation rate along the layer with respect to the paleo-geographiccoordinates; and displaying the sedimentary attribute of the layer in apresent-day geological space.
 2. The method of claim 1, wherein thesedimentary attribute includes sedimentary expansion, which representsvariations in a flux of sediment being deposited.
 3. The method of claim1, wherein the sedimentary attribute is sedimentary potential, whereinthe gradient of the sedimentary potential represents a vector fieldrelated to the lateral variation in the layer's thickness.
 4. The methodof claim 1, wherein the sedimentary attribute is sedimentaryacceleration, wherein the sedimentary acceleration has a magnitudeproportional to the rate of variation in the thickness of the layer anda direction oriented toward the direction of increasing layer thickness.5. The method of claim 1, wherein the sedimentary attribute is selectedfrom a group consisting of: sedimentary acceleration, sedimentaryexpansion, sedimentary potential, sedimentary expansion ratio,sedimentary horizontal divergence, sedimentary horizontal norm,curvature of an IPG-line, curvature of an N-line and normal divergence.6. The method of claim 1, wherein the paleo-geographic coordinates varyalong a surface that follows the curvature of horizons bounding thelayer.
 7. The method of claim 1, wherein the lateral variation dependson derivatives of the sedimentation rate with respect to thepaleo-geographic coordinates.
 8. The method of claim 1, wherein thesedimentation rate accounts for a compaction of sediment describing adecrease in porosity due to a weight of overlying layers of sediment. 9.The method of claim 1, comprising transforming a function of thesedimentary attribute in the present-day geological space to a functionof the sedimentary attribute at a time period when the layer wasoriginally formed.
 10. The method of claim 1, comprising transforming afunction of the sedimentary attribute at a time period when the layerwas originally formed to a function of the sedimentary attribute in thepresent-day geological space.
 11. A system for visualizing sedimentaryattributes, the system comprising: a memory to store a set ofpaleo-geographic coordinates; a processor configured to: receivepaleo-geographic coordinate functions representing predicted approximatepositions of particles of sediment deposited at a time period when alayer was originally formed, determine a sedimentation rate that varieslaterally along the layer, and determine a sedimentary attribute basedon the lateral variation of the sedimentation rate along the layer withrespect to the paleo-geographic coordinates; and a display fordisplaying the sedimentary attribute of the layer in the present-daygeological space.
 12. The system of claim 11, wherein the sedimentaryattribute is sedimentary expansion, which represents variations in aflux of sediment being deposited.
 13. The system of claim 11, whereinthe sedimentary attribute is sedimentary potential, wherein the gradientof the sedimentary potential represents a vector field related to thelateral variation in the layer's thickness.
 14. The system of claim 11,wherein the sedimentary attribute is sedimentary acceleration, whichrepresents the acceleration with which the particles of sediment weredeposited.
 15. The system of claim 11, wherein the processor is totransform a function of the sedimentary attribute in the present-daygeological space to a function of the sedimentary attribute at a timeperiod when the layer was originally formed.
 16. A method for displayingsedimentary attributes, the method comprising: mapping a model of asubsurface layer bound between horizons from a non-curvilinear firstcoordinate system to a curvilinear second coordinate system; determininga sedimentation rate that varies laterally along the layer; determiningone or more sedimentary attributes based on the lateral variation of thesedimentation rate within the layer by approximating the change in thesedimentation rate along one or more iso-surfaces of the curvilinearsecond coordinate system; and displaying the sedimentary attributes inthe model in the first or second coordinate systems.
 17. The method ofclaim 16, wherein the one or more sedimentary attributes comprisesedimentary expansion, which represents variations in a flux of sedimentbeing deposited.
 18. The method of claim 16, wherein the one or moresedimentary attributes comprise sedimentary potential, wherein thegradient of the sedimentary potential represents a vector field relatedto the lateral variation in the layer's thickness.
 19. The method ofclaim 16, wherein the one or more sedimentary attributes comprisesedimentary acceleration, which represents the acceleration with whichthe particles of sediment were deposited.
 20. The method of claim 16,wherein the lateral variation depends on derivatives of thesedimentation rate with respect to the paleo-geographic coordinates.